The present invention relates to a ring trip detector for use in a subscriber line interface circuit; that is, a device for detecting the off hook condition when a ring voltage has been applied to a subscriber line.
In conventional communication networks the subscriber sets (e.g., telephones, data transmitting stations and the like) communicate with the exchange or "switch" via two-wire subscriber lines. Since the connections through the exchange are normally made with two separate branches for respectively transmitting to and receiving from the subscriber sets (that is "four-wire lines") the subscriber line interface circuits which couple the subscriber line with the exchange each have a "hybrid circuit" for the two-wire/four-wire transition. It is the function of such a hybrid circuit to make the signal transfer from the two-wire subscriber line, utilized for both transmission directions, to the transmitting branch of the four-wire line and, vice-versa, from the receiving branch of the four-wire line to the two-wire subscriber line. It is another function of the hybrid circuit to detect when the associated subscriber set is in the "off-hook" condition; that is, when the subscriber line with the subscriber set connected thereto forms a closed loop circuit. When the subscriber set is in the "on-hook" condition, its hook switch opens the circuit between the two wires of the subscriber line.
In addition to making the two-wire/four-wire transition and detecting whether the associated subscriber set is on-hook or off-hook, a hybrid may have the additional function of detecting when a "ground key" button on the subscriber set is pressed. Such ground key buttons are commonly used as signalling devices on subscriber sets (e.g., telephones) in Europe.
Conventionally, in order to ring a subscriber set a "ring voltage" of prescribed frequency and voltage is applied to the subscriber line. For a telephone system, a typical ring voltage may have an RMS value of 95 V and a frequency of 20 Hz. In order to protect the hybrid against this relatively high voltage, a mechanical "ring relay" is provided in the subscriber line interface circuit to connect the subscriber line either to the hybrid or to the ring voltage generator. This relay thus disconnects the hybrid from the two-wire subscriber line when a ring voltage is applied to the line.
One aspect of this arrangement is that the off-hook detector in the hybrid circuit is disconnected and becomes inoperative during the process of ringing. The subscriber line interface circuit thus requires a "ring trip detector" for detecting when someone "picks up" the ringing subscriber set, placing the subscriber set in the off-hook condition. When this state is reached, the "ring trip detector", as it is called, switches the ring relay back to its normal position, disconnecting the ring generator and connecting the hybrid to the two-wire subscriber line. Once the connection between subscriber line and hybrid is made, the off-hook detector within the hybrid becomes operative again and the ring trip detector is not used until the ringing process is again initiated.